Fiber Bragg gratings (FBGs) are commonly used in fiber optic systems for measuring physical quantities such as temperature, pressure and strain at a plurality of sensors. A number of different approaches have been used for the interrogation of FBGs, that is, for determining the center wavelength of the reflection spectrum of each FBG. However, the prior art approaches have several shortcomings that render their usefulness limited for practical applications. Some of these shortcomings include:                Limited number of FBGs that can be interrogated along a single optical fiber        Instability of optical source output properties        Wavelength errors due to stable optical source ripple phase        Susceptibility to intensity and spectrum changes in components that can cause wavelength errors        Susceptibility to power supply noise        Wavelength errors due to low frequency receiver output drift        Errors due to multi-FBG source spectrum masking        Wavelength uncertainty due to low signal-to-noise resulting from very short analog-to-digital conversion times.        